1. Field of the Invention
The present invention relates to an information recording method and apparatus for optical recording media having two information recording layers whose states can be varied by optical irradiation.
2. Background Art
The capacity of visual content recording media, such as optical discs represented by DVDs, is increasing, and an optical disc is being developed that is capable of recording over two hours of high-definition television (HDTV) pictures on a 120 mm-diameter disc. An optical disc format called Blu-ray Disc has lately been established which realizes a recording capacity of approximately 25 GB on one side of the disc by increasing its recording capacity by as much as about five times that of a DVD by employing a semiconductor laser of 405 nm wavelength and an objective lens with a numerical aperture of 0.85. A video recorder adapted for that format has also been put on the market. The Blu-ray Disc specifications are described in “System Description Blu-ray Disc Rewritable Format Part 1: Basic Format Specifications Version 1.0.”
The capacity of optical discs can be increased by increasing their recording density, for example. However, reductions in optical spot size are nearing a limit due to limitations in laser wavelength and the objective lens, and there is little prospect for significant improvements in recording density.
In another method of achieving greater capacity apart from the improvement in recording density, the recording layer is divided into multiple layers. A multilayer optical disc has already been put on the market in the form of read-only optical discs or DVD-ROM discs that have two layers on either one side or both sides (for a total of four layers). There has been no recordable multilayer optical disc as a product so far but specifications for a double-layer rewritable disc in accordance with the Blu-ray Disc format have recently been established for the first time.
FIG. 2 shows the setup of a double-layer recordable optical disc system according to the conventional Blu-ray Disc format. A double-layer disc medium 101 is irradiated with laser light 108 emitted by a laser diode 107 to write or read information. The laser light has the wavelength 405 nm and is focused on a recording layer by an objective lens 112 with numerical aperture (NA) of 0.85, thus forming an optical spot of a diameter of about 400 nm, with which information is written or read. During the writing of information, a high-power laser light is shone in order to cause a physical change in the recording layer material, such that long and short recording marks are formed. A minimum recording mark length is 160 nm, and approximately 23.3 GB of information per recording layer, or approximately 46.6 GB of information can be recorded on a single double-layer optical disc.
The double-layer optical disc medium 101 is a disc with a diameter of 120 mm and a thickness of 1.2 mm and is made up of a substrate 102, a first recording layer (L0 layer) 103, a spacer layer 104, a second recording layer (L1 layer) 105 and a cover layer 106 sequentially stacked upon one another. The substrate 102 is a disc of thickness 1.1 mm and is made of polycarbonate resin or other material. On the surface of the substrate is formed a spiral guide groove (track) with a 0.32 μn pitch for allowing the optical spot to trace at a certain radial position (tracking). The recording marks are formed within the guide groove. The first recording layer (L0 layer) 103 is formed next to the substrate. The recording layer is normally made of a recording film disposed between protective layers, with the recording film being made of a phase-change material, such as a GeSbTe alloy or an AgInSbTe alloy. Regardless of whichever recording film material is used, binary data is recorded by assigning its “0's” and “1's” to a high-reflectance crystal portion and a low-reflectance amorphous portion, respectively, and the data is played back by identifying the “0's” and “1's” of the binary data based on their difference in reflectance. Next to the L0 layer 103 is formed the second recording layer (L1 layer) 105 via the spacer layer 104, which is formed to separate the first recording layer 103 and the second recording layer 105. The spacer layer 104 has a thickness of about 25 μm and is made of a UV curable resin. The structure of the L1 layer 105 is substantially identical to that of the L0 Layer 103. Finally, the cover layer 106, which has a thickness of 75 μm and is made of a UV curable resin, is formed to provide protection for the surface of the disc.
In the double-layer optical disc, however, during recording in the L0 Layer, which is located farther from the side on which the light is incident, different levels of laser power are delivered to the L0 Layer through a recorded region and an unrecorded region on the L1 layer, which is located before the L0 Layer, due to the effective difference in transmittance of the L1 layer.
To solve this problem, JP Patent Publication (Kokai) No. 2003-109217 (Patent Document 1) discloses the difference in transmittance between the unrecorded and recorded portions in the L1 layer is limited, such that the L0 Layer can be recorded with a certain write power regardless of the state of recording on the L1 layer. Another method is disclosed in JP Patent Publication (Kokai) No. 2002-279634 (Patent Document 2) whereby the write power directed to the L0 layer is controlled in accordance with the level of a read RF signal from the L0 layer.
Patent Document 1: JP Patent Publication (Kokai) No. 2003-109217
Patent Document 2: JP Patent Publication (Kokai) No. 2002-279634
Non-Patent Document 1: System Description Blu-ray Disc Rewritable Format Part 1: Basic Format Specifications Version 1.0